Method for joining dissimilar materials

ABSTRACT

A composition of matter includes a first compatible material having particles containing chemical elements similar to a first substrate, and second compatible material having particles containing chemical elements similar to a second substrate, wherein the first substrate and the second substrate are chemically different. The particles are dispersed into a matrix that is in between the first and the second substrate. A deposition system has a multi-material printhead, a first reservoir of a first compatible material having particles containing chemical elements similar to a first substrate, a second reservoir of a second compatible material having particles containing chemical elements similar to a second substrate, a third reservoir of an polymer precursor material, and at least one mixer. A method of bonding a joint between dissimilar substrate materials includes functionalizing a first compatible material having chemical elements similar to a first substrate, mixing the first compatible material with a polymer precursor material, functionalizing a second compatible material having chemical elements similar to a second substrate, mixing the second compatible material with a polymer precursor material, and using the deposition system to deposit the first and second compatible materials and a polymer precursor material on the joint between the first and second substrate materials.

TECHNICAL FIELD

This disclosure relates to joining dissimilar materials, moreparticularly joining metals and glass with polymer composite materials.

BACKGROUND

One of the outstanding challenges in the adhesive industry is bonding ofdissimilar materials. An adhesive can be optimized for bonding identicalparts such as glass-to-glass or carbon fiber reinforced polymer (CFRP),CFRP-to-CFRP but they provide only poor sealing and adhesion betweendifferent materials, i.e. glass-to-CFRP. This is because of thedifferent chemical nature and surface free energy properties of thesedifferent materials. The problem is particularly relevant for theautomotive industry where the introduction of lightweight materials ischallenged due to the absence of materials and processes to createmechanically robust and corrosion free load-bearing structures. Examplesinclude joining dissimilar materials such as any combination of thefollowing: carbon fiber reinforced polymer composite (CFRP), fiber glassreinforced polymer (FRP) composite, aluminum, glass, titanium andmagnesium. Structural adhesives provide low-cost, corrosion-free bondsand eliminate the need for failure-prone through-holes in CFPCstructures, currently used to bond these materials.

For example, leading adhesives such as 3M®'s 460 or Dow Chemical®'sBetaforce®, provide highly effective bonding identical aluminum parts,with a bonding strength up to 40 mega Pascals (MPa). Yet, when bondingaluminum to CFRP parts, the performance of these same adhesives is poor(15 MPa). The CFPC side of the bond requires significant improvement.

It becomes difficult to identify single regular adhesives that providestrong bonding of dissimilar materials.

SUMMARY

According to aspects illustrated here, there is provided a compositionof matter includes a first compatible material having particlescontaining chemical elements similar to a first substrate, and secondcompatible material having particles containing chemical elementssimilar to a second substrate, wherein the first substrate and thesecond substrate are chemically different. The particles are dispersedinto a matrix that is in between the first and the second substrate.

According to aspects illustrated here, there is provided a depositionsystem has a multi-material printhead, a first reservoir of a firstcompatible material having particles containing chemical elementssimilar to a first substrate, a second reservoir of a second compatiblematerial having particles containing chemical elements similar to asecond substrate, a third reservoir of an polymer precursor material,and at least one mixer.

According to aspects illustrated here, there is provided a method ofbonding a joint between dissimilar substrate materials includesfunctionalizing a first compatible material having chemical elementssimilar to a first substrate, mixing the first compatible material witha polymer precursor material, functionalizing a second compatiblematerial having chemical elements similar to a second substrate, mixingthe second compatible material with a polymer precursor material, andusing the deposition system to deposit the first and second compatiblematerials and a polymer precursor material on the joint between thefirst and second substrate materials

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art example of a bond between dissimilar materials.

FIG. 2 shows an embodiment of a bond between dissimilar materials.

FIG. 3 shows an embodiment of a method of forming a gradient adhesivebond with dissimilar materials.

FIG. 4 shows a block diagram of an embodiment of a gradient adhesiveprint head.

FIG. 5 shows an embodiment of a slot die print head suitable fordepositing gradient adhesives

FIGS. 6-9 shows different bonding techniques.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The ensuing discussion focuses on bonding combinations of differentmaterials such as aluminum, CFRP, glass, fiberglass reinforced polymers(FRP), titanium and magnesium parts, such as commonly occurs in theautomotive manufacturing environment. One should understand that thisprovides only examples and do not limit the invention as claimed to anypair of dissimilar materials.

The embodiments here provide a high-performance structural adhesive forbonding dissimilar materials with the following key features. First, theadhesive has direct chemical bonding to each of the dissimilarmaterials. Second, the adhesive has a graded composition of chemicallyand mechanically compatible reinforcing particles with a tunableadhesive elasticity that enables modulation of the particleconcentration during deposition. Third, chemically linked networks bycovalent bonds, of reinforcing particles that prevent particleseparation during impact and stress.

In the embodiments, functionalized particles are dispersed into amatrix. In embodiment, the matrix may consist of polymer precursormaterials. Covalent chemically linked particle networks are formed byreacting the chemical functions present onto the particles surface withthe polymer precursors to produce a cured structure.

In current bonds of dissimilar materials, as shown in FIG. 1, polymerbonds are formed between the two parts. The polymer bonds to the partswith a materials mismatch, and has free dispersed particles. Thisresults in weak bonding. The bonds, such as at 12 and 16 at theinterfaces between the adhesive and the materials, are the same as theyare in regions such as 14 of the adhesive away from the materials.

Direct particle bonding to surfaces results in superior adhesivestrength. Current materials achieve their adhesive strength only by thebonding of polymer chains to the surfaces. In the embodiments here,matching particles will directly and covalently bond onto the surface ofthe joined substrates. Chemical linking forms the bond, or chemical andphysical linking simultaneously form the bond in the case of particles.Direct particle bonding to the surface will result from reacting surfacefunctionalized particles with the activated surface using epoxidemonomers.

When dissimilar materials bond, it is not sufficient to simply having astrong bond for durability. The bond needs to act to absorb differencesin material properties between the two parts. Graded interfaces betweendifferent materials can reduce the shear stress at an interface andallow two dissimilar surface to become compatible. This approach canserve to grade mechanical properties between the two materials, as wellas to bridge the two materials chemically, allowing for increasedbonding.

The effectiveness of the gradient approach to produce enhanced boundinghas been shown recently in plastic materials of different stiffness.Studart has bonded hard structures onto a stretchable substrate, andthen stretched them up to 350% before delamination occurred (A. R.Studart, et al. Nature Communications, 3, 1265 (2102)). Homogenousadhesive bonding failed rapidly at about 200% strain. Using a multilayergraded interface between the dissimilar hard and soft substrates greatlyreduced the interlayer shear forces and prevented delamination. Thisshowed an overall increase of the strain level before delamination ofapproximately 75% when compared to homogenous bonding.

Conventional adhesives rapidly lose their bonding performance as theenvironmental temperature increases. For example, 3M's 460 epoxy lapshear strength decreases from 30 MPa at 23° C. to 5 MPa at 82° C. andfurther decreases to 1.5 MPa at 121° C.(http://multimedia.3m.com/mws/media/661220/3mtm-scotch-weld-tm-epoxy-adhesive-dp460-ns-and-off-white.pdf).The cohesive strength of the adhesive at higher temperatures decreasessignificantly due to the softening of polymer chains as temperaturesincrease. To improve cohesion at high temperatures, the use ofreinforcing particles provides a viable strategy. However, whileindividual reinforcing particles themselves possess exceptionalmechanical strength, translating these properties into macroscopicallystrong composite structures has not yet been demonstrated. For example,exfoliated graphene particles have outstanding tensile strength(130,000) MPa (Novoselov at al. Nature, 490, 192-200 (2012)). Thetensile strength decreases by orders of magnitude (250 X) in graphenepolymer composites (M. A. Rafiee et. al., ACS NANO, 3884, (2009).

In addition, the free particles are not bonded to the nearby epoxygroups and deformations due to stress can cause permanent dislocation ofthe particle from the polymer matrix, eventually causing failure. Theyenable only a limited improvement of the cohesive strength when comparedto the polymer material alone. Chemical linking will restrict themovement of the reinforcing particles under mechanical and thermalstresses. As a result, the material should have outstandingthermo-mechanical performance when compared with current adhesives. Oneexpects that the simultaneous enhancement of the cohesive strength, bycovalent bonding of the strong particle networks, and of the adhesivestrength, by covalent bonding of the strong particle network to eachsurface, will reduce the slippage between bonded surfaces. Moreover,this will produce adhesive bonds with mechanical thermal cycling andfatigue resistance far exceeding the performance of current adhesivesfor bonding either identical or dissimilar materials.

The selection of the particles that either match or have similarthermo-mechanical and chemical properties to the respective substratesto be bonded depends on the nature of these materials. This may beachieved by choosing particles that contain chemical elements that aresimilar to the corresponding compatible substrate. The chemicalcomposition of the particles may or may not be identical to thecorresponding compatible substrate. For the purpose of this discussion,“similar chemical elements” means that the particle materials contain atleast the key chemical element(s) present in the correspondingcompatible substrates. The particles materials may contain additionalchemical elements that may not be present in the compatible substrate tobe bonded.

One the embodiment of the compatibilizing particles with compatibilitywith the substrate consist of metal oxides of the same materials as themetal substrates to be bonded. Another embodiment of compatibilizingparticles with compatibility with the substrate consist of carbonparticles when bonding carbon fiber reinforced polymers (CFRP)substrates.

For example, the particles used for reinforcing the polymer base in theadhesive that is to be placed on the side of a selected substrate areselected based upon the following. For a CFRP substrate, graphene,graphene oxide or carbon nanotube functionalized particles are used. Foran aluminum substrate, the compatibilizing particles would consist ofaluminum oxide particles (Al₂O₃) particles. For a fiber glass reinforcedpolymers (FRP) substrate, they would consist of silicon oxide (SiO₂)particles. For glass, the process would also use silicon oxide (Sift)particles. For titanium substrates, the process would use titaniumdioxide (TiO₂). For magnesium substrates, the process would usemagnesium oxide (MgO).

The adhesive will have a gradient of these materials to have higherconcentrations of the material bonding to one of the dissimilarmaterials at the interface to that material, and then lowerconcentrations at the interface to the other material. FIG. 2 shows agradient adhesive bond 20. One should note that in FIG. 2, the firstmaterial substrate could be a metal substrate, glass, CFRP, FRP, etc.,and the second material substrate would be the other material. Thecorresponding compatible particles for the first substrate will bereferred to as the first compatible particles, the correspondingcompatible particles for the second substrate will be referred to as thesecond compatible particles.

Gradients can occur in multiple ways. A simple gradient may alter theconcentration of reinforcement so that the overall concentration ofreinforcement is constant across the entire bond, but the relativeconcentration of each reinforcement changes across the bond so thatconcentration at the two ends is entirely the compatible reinforcement.However, in other cases, the gradient may be more complicated. To addcompliance or reduce corrosion, it may be desirable to have a less stiffsection of the bond between the two components. For example, this may behighly desirable when bonding glass to an aluminum component, in thiscase the overall reinforcement would vary across the bond, reaching itsminimum in the middle of the bond with no or little overlap between thetwo particles. It also may be of interest to provide a gradient acrossthe width of the bond, decreasing the particle concentration at theedges of the bond will increase resistance to some mechanical loadingand may be desirable.

As an example, the bonding of a first substrate as CFRP and a secondsubstrate as an aluminum substrate is illustrated. At the CFRP interface22, the carbon fibers from the CFRP material chemically bond to thegraphene particles as the first compatible particles in the adhesive,with a higher concentration of the graphene at this interface than atthe other, and a higher concentration of graphene than the aluminumoxide as the second compatible particles at this interface. The gradientfor the aluminum oxide goes in the other direction, with a higherconcentration of aluminum oxide at the aluminum interface 26, and alower concentration moving towards the CFPC-adhesive interface. Theregion in the middle, such as 24, has a mix of the two particles.

Multiple adhesive chemistries are suitable for the present invention.Particularly preferred are vinyl, such as acrylic adhesives, and epoxyadhesives. Acrylic adhesives are particularly advantageous in situationswhen fast curing is required. When acrylic adhesives are being used,they require employing vinyl functionalized particles. They can beobtained for example by reacting X—(R)-Vinyl molecule with —COOH or —OHfunctionalized particles. An example of a suitable reactive X group isepoxy group. Another approach to vinyl functionalized particles involvescoupling of reactive particles with silane coupling agents containingfunctional groups such as vinyl, including acrylates, and methacrylates.

Generally speaking, conventional two-part epoxy adhesives consist of apart A, epoxy material, and a part B, hardener. Each of the twoadhesives forming the gradient adhesive bond will be deposited as amixture of its own part A and part B. For each of these two adhesives,the reinforcing particles disclosed in the present invention can bepresent in either of the two corresponding parts A or B. For example, infurther describing the CFRP and aluminum parts bonding embodiment,appropriately functionalized particles can be present as follows in theformulation: graphene particles can be present into the epoxy part A orinto the hardener part B materials. Just prior to deposition, the part Aand B are mixed and the mixture represents the CFRP compatible adhesivematerial. In a similar way, aluminum oxide particles can be placed intothe epoxy part A or into the hardener part B materials. Prior todeposition, the part A and B are mixed and the mixture represents thealuminum compatible adhesive material. These premixed formulations areused to produce the gradient adhesive structure when the ratio betweenthe two mixtures is gradually changed.

The choice of placing the functionalized particles in the epoxy, part A,or the hardener, part B, is dictated by the type of functional groupspresent onto particles. The particles must be stable in the basematerial. For example, functionalized particles in the hardener must notreact with the hardener but must react with the epoxy when thehardener/particle dispersion is mixed with the epoxy. Similarly,functionalized particles in epoxy must not react with the epoxy materialbut must react with the hardener when the epoxy/particle dispersion ismixed with the hardener. Once part A and part B of each of the adhesivesare mixed the curing is initiated by the reaction between the aminogroups and epoxy groups both those present in the base materials andthose present onto the surface of the particles. Depending on the typeof hardener, such as amino component, the curing can be performed at theambient temperature or at higher temperature by heating. Generally, acuring process that may be relatively slow at ambient temperature issignificantly accelerated by curing at a higher temperature.

One embodiment involves functionalized particles dispersible in thehardener material. The hardener materials contain mostly reactive aminogroups. Suitable functionalized particles that are stable anddispersible in hardener material include particles possessing groupssuch as amino (—NH₂, —NHR) groups, alcohol groups (—OH) and carboxylicacid groups (—COOH). A hardener formulation containing such functionalparticles will cure when mixed with the epoxy component, part A, eitherat room temperature or by heating due to the coupling reaction betweenthese functional groups and the epoxy groups, as it is well known tothose skilled in the epoxy adhesives.

One exemplary embodiment involves amino functional groups. The processfor fabrication of amino functionalized particles depends on thestarting particle and more precisely on the coupling functional groupspresent onto particles.

One method for introduction of amino groups onto metal oxide particlessuch as SiO₂, Al₂O₃, TiO₂ or MgO, involves surface functionalization ofmetal oxide particles with amino functionalized silane couplingreagents. Amino silane coupling agents have a general structure such asH2N—(X)—Si(OR)₃ where the —Si(OR)₃ groups that react selectively withthe —OH groups present onto particles surface to produce particlesterminated with amino groups as illustrated in the figure below:

Amino functionalized exfoliated graphene sheets can be fabricated inseveral ways. The process can use any available graphene material as astarting material. This includes graphene, graphite, graphene with anysubstituting or doping chemical elements such as for example graphenecontaining oxygen functions, such as in the case of graphene oxide.Graphene with other chemical elements is also suitable for fabricationof amino functionalized exfoliated graphene sheets. One process asdisclosed by Choi, et al. (Chem. Commun., (2010), 46, 6320-2).Exfoliation is a process of turning three-dimensional molecules intotwo-dimensional sheets. In this process, amino functionalized sheets arefabricated through a mild Friedel-Crafts substitution reaction ontographite particles with simultaneous exfoliation of amino functionalizedgraphene sheets. Another approach to producing amino functionalizedgraphene sheets starts from graphene oxide (GO) sheets. Graphene oxidecontains a large content of —COOH and —OH functional groups that can beused for introduction of amino functional groups, for example byreacting GO with molecules of the general structure X—(R)—NH2 where Xhas a selective reaction with —OH or —COOH groups in the presence of acatalyst. Suitable X groups include —NH₂ and silane coupling agents(—Si(OR)₃) groups. Other methods for producing amino-functionalizedgraphene particles may be available and they are all suitable for thisinvention.

In a further embodiment, the functionalized particles are dispersed inthe hardener, part B, the reactive functional groups of are either —COOHor —OH groups. Most often, particles such as SiO₂, Al₂O₃ or TiO₂ havehydroxyl groups present onto their surface. In the case of graphenesheets, a suitable particle is graphene oxide that contains a largecontent of —COOH and —OH functional groups.

A second embodiment involves functionalized particles dispersed in theepoxy material (part A). Suitable examples include epoxy functionalizedparticles because they are stable and compatible with epoxy basematerials. Epoxy functionalized particles can be fabricated by reacting—OH or —COOH functional groups present onto the surface of the particleswith epoxy containing reagents of a general structure X—(R)-Epoxy whereX reacts selectively with —OH and —COOH groups. The process isapplicable to any type of —OH and —COOH functionalized particles,including Al₂O₃, SiO₂, TiO₂, MgO and similar as well as graphene oxide.A preferred reactive X function is an epoxy group.

In the embodiment of aluminum bonding aluminum to CFPC, the resultingbond has a higher concentration of aluminum oxide than graphene at thealuminum interface and a higher concentration of graphene than aluminumoxide at the CFPC interface. In a more general sense, the compound hastwo materials as hardeners in an epoxy resin, where each material has agraded composition of a higher concentration at one of the substratesmoving towards a lower concentration at the other substrate, and the twomaterials have graded compositions in the opposite direction.

In a different embodiment with epoxy functionalized particles involves atype of one part epoxy formulations wherein an epoxy material is mixedwith epoxy functionalized particles and with a curing initiator that isactivated only when heated at a certain temperature. This type offormulation is advantaged by the fact that the formulation can be stablein that is does not cure and can be shipped and handled for long periodsof time before application. This formulation is also advantageousbecause it reduces the number of mixing steps require for each of thepart A and B of each of the adhesives using amino hardeners. Afterdeposition the curing takes place by heating at a temperature above theambient temperature such as at 80 deg. C. or 100 deg. C. or higher forvarious time durations such as as from minutes to hours. Optionally, asecond heating step can be performed at a higher temperature, such as160° C. or 200° C. or 220° C. This process ensures curing completion andgenerally produces stiffer bond when compared with curing at only lowertemperatures.

The present approach is generally applicable to bonding any twodifferent materials. For example, when bonding CFRP to glass, the CFRPside of the adhesive layer contains functionalized graphene particleswhile on the glass side the adhesive contains SiO₂ functionalizedparticles. When bonding aluminum to glass, the aluminum side of theadhesive layer contains functionalized Al₂O₃ particles while on theglass side the adhesive contains SiO₂ functionalized particles.

FIG. 3 shows an embodiment of a method of depositing a gradient adhesivefor dissimilar materials using epoxy embodiment. The examples used hereillustrate the use of amino functionalized particles dispersed in ahardener when bonding CFRP and aluminum. The process uses exfoliatedamino graphene 40 and amino aluminum oxide 42 particles. At 44 Aminographene is mixed with hardener material to provide a graphene hardener,which is then mixed in the next step with epoxy material 48. Similarly,amino Al₂O₃ at 42 is first mixed with the hardener to provide an Al₂O₃hardener at 46 which is then mixed in the next step with epoxy materialat 50. The hardener and epoxy materials may be the same for each of thetwo adhesive particles.

These materials are provided to the printhead. A printhead, in thecontext of the present invention may include any extrusion andcoextrusion deposition device, such as slot die or conventionalextrusion through a needle. The following examples illustrate thedeposition through a slot die.

Successive layers ranging from a first layer of pure aminographene/epoxy mixture 52, up to a last layer consisting of pure aminoAl₂O₃/epoxy formulation 56 with intermediate gradual decreasingconcentration of the amino graphene/epoxy and increasing concentrationof amino Al₂O₃/epoxy 54 are deposited through a slot die. The aluminumsubstrate is placed on top of the deposited gradient adhesive 58 and thebonded structure is cured 59.

To achieve deposition rates fast enough for automotive applications, aprint head capable of producing gradient adhesive bonds can deposit theadhesive in a single pass. Traditional automated automotive adhesivedispensers consist of a single needle point application mounted on thefront of a robot with a single material feed system. The robot haseither undergone pre-programming or uses a vision system to locate theappropriate application point and dispense the needed amount ofadhesive. FIG. 4 5 show an embodiment of a slot die print head.

In FIG. 4, the slot die print head 60 has material feeds 62 of theepoxy, the graphene and hardener, and the aluminum oxide and hardener. Apump 64 typically provides the pressure to move the materials. The mixeror mixers 66 receive the materials from the material feeds and providethe desired concentrations for the adhesive bond being formed. Themanifolds 68 then merge the feeds, and the die itself 70 deposits theadhesive across the interface between the dissimilar materials to formthe bond. FIG. 5 shows an image of such a print head.

The print head can deposit small numbers of layers by premixing formulasand depositing all three layers simultaneously. The ideal graded bondmay require larger numbers of layers. To accomplish multiple layerssimultaneously, the print head may mix the epoxy formulations, meaningthe combination of hardeners and resin, within the print head. This alsoallows for changes in gradient composition if necessary, customizing thegradient for a specific bond. This may reduce equipment costs andmaximize flexibility in automotive factories.

FIGS. 6-9 show variations of the different bonds. FIG. 6 shows anembodiment of a bond in accordance with the traditional bondingtechniques. A monolithic layer of adhesive 62 bonds a substrate of afirst material, such as carbon fiber, 60 and a substrate of a secondmaterial, such as aluminum 64. There are sharp changes in the propertiesbetween the various layers.

In contrast, the embodiments discussed here have gradients within a basematerial of particles of compatible materials. In FIG. 7, a highrigidity bond has a particle concentration kept high in the adhesivelayer 66 allowing the mechanical properties to transition from grapheneto aluminum oxide.

FIG. 8 shows a flexible gradient bond to compensate for dimensional orthermal differences. The adhesive layer 68 has a flexible gradient bondwith a lower concentration of particle reinforcement, allowing forflexibility while maintaining gradual changes in properties such asthermal or dimensional differences.

FIG. 9 shows an adhesive layer 70 that has a particle gradient in theplane of the bond. This allows for an increase in the overall bondstrength.

In this manner, the embodiments achieve a structural adhesive that hadenhanced cohesive strength enabled by chemically linked networks ofreinforcing particles that prevent particle motion during impact andstress. The adhesive has maximized strength due to direct chemicalbonding of matched particles to the surfaces. The embodiments create anenhanced interface between dissimilar materials with the gradedcomposition of chemically and mechanically compatible reinforcingparticles. These embodiments easily integrate with current roboticdeposition systems with the replacement of the current nozzles with amulti-material slot die.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A multilayer structure comprising: a firstsubstrate; a second substrate chemically different from the firstsubstrate; a curable matrix between the first and second substrate;particles of a first compatible material functionalized with chemicalfunctions capable of reacting with the curable matrix during curing toform a network of chemically linked particles, the particles containingchemical elements similar to the first substrate; and particles of asecond compatible material functionalized with chemical functionscapable of reacting with the curable matrix during curing to form anetwork of chemically linked particles, the particles being chemicallydifferent from the particles of first compatible material and containingchemical elements similar to the second substrate; wherein the particlesof first compatible material are dispersed in the curable matrix and arepresent in a first concentration gradient where the concentrationincreases in a region approaching the first substrate; and wherein theparticles of second compatible material are dispersed in the curablematrix and are present in a second concentration gradient where theconcentration increases in a region approaching the second substrate. 2.The multilayer structure of claim 1, wherein the first substrate and thesecond substrate are selected from the group consisting of: carbon fiberreinforced polymer; fiber glass reinforced polymer; glass; aluminum;magnesium; and titanium; and the first substrate and the secondsubstrate are not the same.
 3. The multilayer structure of claim 1,wherein the curable matrix is an acrylic adhesive or a two-part epoxyadhesive consisting of an epoxy material and a hardener material.
 4. Themultilayer structure of claim 1, wherein the particles of firstcompatible material and the particles of second compatible material areselected from the group consisting of: graphene; graphene oxide; carbonnanotubes; aluminum oxide; titanium dioxide; silicon dioxide andmagnesium dioxide; provided that the particles of first compatiblematerial and the particles of second compatible material are not thesame and are functionalized with functions capable of reacting with thecurable matrix during curing to form a network of chemically linkedparticles.
 5. The multilayer structure of claim 1, wherein the firstsubstrate is aluminum and the second substrate is carbon fiberreinforced polymer composite (CFPC).
 6. The multilayer structure ofclaim 5, wherein the particles of first compatible material arefunctionalized aluminum oxide and the particles of second compatiblematerial are functionalized graphene or graphene oxide.
 7. Themultilayer structure of claim 1, wherein the particles of firstcompatible material and the particles of second compatible material arepresent in an overall concentration that is uniform throughout thecurable matrix and the first gradient and the second gradient smoothlytransition from the first substrate to the second substrate.
 8. Themultilayer structure of claim 1, wherein the particles of firstcompatible material and the particles of second compatible material arepresent in an overall concentration, from first substrate to secondsubstrate, that is highest in the regions approaching the first andsecond substrates and reduced at approximately the midpoint of thecurable matrix between the first and second substrates.
 9. Themultilayer structure of claim 8, wherein at least one of the particlesof the first compatible material and the particles of the secondcompatible material is present at lower concentrations at the edges ofthe curable matrix.
 10. The multilayer structure of claim 1, wherein atleast one of the particles of the first compatible material and theparticles of the second compatible material is present at lowerconcentrations at the edges of the curable matrix.
 11. A composition ofmatter, comprising: a first substrate; a curable matrix on the firstsubstrate, wherein the curable matrix has an exposed bonding surface;particles of a first compatible material functionalized with chemicalfunctions capable of reacting with the curable matrix during curing toform a network of chemically linked particles, the particles containingchemical elements similar to the first substrate; and particles of asecond compatible material functionalized with chemical functionscapable of reacting with the curable matrix during curing to form anetwork of chemically linked particles, the having particles beingchemically different from the particles of the first compatible materialand containing chemical elements similar to a second substrate, which isto be contacted with the bonding surface of the curable matrix; whereinthe particles of first compatible material are dispersed in the curablematrix and are present in a first concentration gradient where theconcentration increases in a region approaching the first substrate; andwherein the particles of second compatible material are dispersed in thecurable matrix and are present in a second concentration gradient wherethe concentration increases in a region approaching the bonding surfaceof the curable matrix.
 12. The composition of matter of claim 11,wherein the curable matrix is an acrylic adhesive or a two-part epoxyadhesive consisting of an epoxy material and a hardener material. 13.The composition of matter of claim 11, wherein the particles of firstcompatible material and the particles of second compatible material areselected from the group consisting of: graphene; graphene oxide; carbonnanotubes; aluminum oxide; titanium dioxide; silicon dioxide andmagnesium dioxide; provided that the particles of first compatiblematerial and the particles of second compatible material are not thesame and are functionalized with functions capable of reacting with thecurable matrix during curing to from a network of chemically linkedparticles.
 14. The composition of matter of claim 11, wherein theparticles of first compatible material are functionalized aluminum oxideand the particles of second compatible material are functionalizedgraphene or graphene oxide.
 15. The composition of matter of claim 11,wherein the particles of first compatible material and the particles ofsecond compatible material are present in an overall concentration thatis uniform throughout the curable matrix and the first gradient and thesecond gradient smoothly transition from the first substrate to thebonding surface.
 16. The composition of matter of claim 11, wherein theparticles of first compatible material and the particles of secondcompatible material are present in an overall concentration, from firstsubstrate to bonding surface, that is highest in the regions approachingthe first substrate and the bonding surface, and is reduced atapproximately the midpoint of the curable matrix between the firstsubstrate and the bonding surface.
 17. The composition of matter ofclaim 11, wherein at least one of the particles of the first compatiblematerial and the particles of the second compatible material is presentat lower concentrations at the edges of the curable matrix.
 18. Acomposition of matter, comprising: a first substrate; a curable matrixof a two-part epoxy adhesive consisting of an epoxy material and ahardener material, wherein the curable matrix has an exposed bondingsurface; particles of a first compatible material functionalized withchemical functions capable of reacting with the epoxy material or thehardener material during curing to form a network of chemically linkedparticles, the particles containing chemical elements similar to thefirst substrate; and particles of a second compatible materialfunctionalized with chemical functions capable of reacting with theepoxy material or the hardener material during curing to form a networkof chemically linked particles, the particles being chemically differentfrom the particles of the first compatible material and containingchemical elements similar to a second substrate, which is to becontacted with the bonding surface of the curable matrix; wherein theparticles of first compatible material are dispersed in the curablematrix and are present in a first concentration gradient where theconcentration increases in a region approaching the first substrate; andwherein the particles of second compatible material are dispersed in thecurable matrix and are present in a second concentration gradient wherethe concentration increases in a region approaching the bonding surfaceof the curable matrix.
 19. The composition of matter of claim 18,wherein the particles of first compatible material and the particles ofsecond compatible material are functionalized with amino groups and areinitially dispersed in the hardener material.
 20. The composition ofmatter of claim 18, wherein the particles of first compatible materialand the particles of second compatible material are functionalized withhydroxyl groups, carboxyl groups or a combination of hydroxyl andcarboxyl groups and are initially dispersed in the hardener material.21. The composition of matter of claim 18, wherein the particles offirst compatible material and the particles of second compatiblematerial are functionalized with epoxy groups and are initiallydispersed in the epoxy material.